1. Field of the Invention
Embodiments of the present invention generally relate to a charge-pump for use in a micro electromechanical system (MEMS) device.
2. Description of the Related Art
A digital variable capacitor (DVC) operates with electrostatic forces. In this mechanism a force is acting on the moveable MEMS element when a voltage V is applied between the MEMS element and a control electrode. This electrostatic force scales with (V/gap)2. The mechanical counter-balance force comes from a spring suspension system and typically scales linearly with the displacement. The result is that with an increasing voltage V the MEMS device moves a certain distance δ toward the control-electrode. This movement reduces the gap which in turn increases the electrostatic force further. For small voltages, an equilibrium position between the initial position and the electrode is found. However, when the voltage exceeds a certain threshold level (the pull-in voltage), the device displacement is such that the electrostatic force rises faster than the mechanical counterbalance force and the device rapidly snaps-in towards the control-electrode until it comes in contact.
Some DVC devices have a control-electrode above (i.e., a pull-up or pull-off or PU-electrode) and below (i.e., a pull-down or pull-in or PD-electrode) the moveable MEMS element, as shown schematically in FIG. 1. In addition there is an RF-electrode below the moveable MEMS element. During operation the MEMS element is either pulled-up or pulled-down in contact to provide a stable minimum or maximum capacitance to the RF-electrode. In this way the capacitance from the moveable element to the RF-electrode (which resides below the moveable element) can be varied from a high capacitance Cmax when pulled to the bottom, as shown in FIG. 2, to a low capacitance Cmin when pulled to the top, as shown in FIG. 3.
The voltages applied to the PD-electrode (Vbottom) and to the top-electrode (Vtop) typically range from 10V to 40V, which is much larger than the system-level supply voltage VDD which typically ranges from 1.8V to 3.3V.
Therefore, there is a need for an on-chip charge-pump which boosts the voltage levels from VDD to HV.